Book of Spirits, Part 1: The Demon Rivaille
by Smellybelly69
Summary: Supernatural AU. Sometimes a deal with a demon is better than no deal at all. Warnings: Explicit sex, slavery and violence.
1. Prologue

**Prologue**

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**Book of Spirits, Page 4**  
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**de·mon** [**dee**-m_uh_n]

_**noun**_

An evil spirit; devil or fiend that exists within one of the seven circles of hell. They mainly deal with humans, fulfilling desires and wishes in exchange for life span (it should be noted that servitude in the afterlife is in most cases an acceptable substitute).

After the great sacrifice of 1583 _(see page 53),_ the great demon Buer put in place ten demonic laws, although these laws were later revised by The Council of Four in 1701 and it is now accepted that most demons are only required to follow the first three. These three laws are as follows;

_Demons may deal with any human* provided the motive is selfish._ They may offer any service to a human so long as the human's desire corresponds with one of the seven deadly sins. Deals that may incite a heroic sacrifice, or save the life of a loved one are expressly forbidden.

_Demons cannot befriend humans. _The transaction between a demon and a human is purely business. Whilst a demon may take a _Servus_ human (an owned human) to use for pleasure, they _cannot_ become attached.

_Demons must obey greater demons. _This includes any demon from a lower level of hell and _especially_ those under the direct order of H.I.M (His Infernal Majesty).

Failure to obey these rules can and will result in punishment. This may include readmission to a higher level of hell, torture and in extreme cases death.

*Demons are discouraged from dealing or attempting to deal with those under divine protection (e.g. anyone with a guardian angel), lest they face the eternal wrath of heaven.

* * *

"We're going to die."

Eren grinned, "Not if we kill _it_ first."

"How do you know you'll have to kill anything?" Mikasa asked with a smirk, "This exam isn't only a test of skill; it's a test of _character_."

This didn't seem to reassure Armin, who moved his shaking arms upwards to press the book he was holding tightly against his chest. "I don't know about this Eren, I mean we can always retake the exam next year…"

"No way! We've been training for over three years now; I'm not waiting any longer to become a Reaper."

"The Reapers are a group of elite assassins used to take down dangerous supernatural beings! The hardest things we've faced so far are a couple of lesser spirits and a possessed clock." Armin ran a hand through his hair and frowned. "I'm sorry Eren…I'm just so _afraid_. Once the coffin door closes there's no going back!"

The exam was situated in a cemetery because they are considered to be the point where the barrier between this world and the afterlife is at its thinnest. Reapers graves are dug for them when they decide to join up and following this each reaper has their own coffin made pre-made and placed inside of the grave. The coffins are then enchanted so they may be used as a way to contact those from other worlds and they eventually become the main way in which reapers solicit potential employers.

"We've been trained by _Mikasa_, she wouldn't let us do the exam if she didn't think we could handle it." Eren inclined his head towards the dark haired girl who acknowledged the compliment with a brief smile. She was by far the most talented of the three and the only full reaper amongst them. She was also their instructor for the exam.

"He's right Armin," She said gently, "I wouldn't have let you enter if I wasn't sure you could pass."

"Besides Armin, you're the smartest guy I've ever met and you've saved my life more times than I can count. If anyone deserves to become a reaper then it's _you_." Eren smiled and placed a hand on his shoulder. "I believe in you."

Armin smiled and clenched his fists in determination. "Okay, then I guess I'm in! What do we have to do?"

Mikasa reached into her top and pulled out a pendant. Held together on a long thread of silver was a round piece of glass and encompassed within it was a perfect golden flower.

"A flower?" Eren laughed, examining the pendant. "Hey Armin, have a look at this."

Armin nodded, inspecting the flower closely, "That's not a regular flower...how long have you had this pendant Mikasa?"

"Since I passed my own exam a year ago."

"And it hasn't wilted." Armin said thoughtfully, "I've read about this type of flower before - this is a golden rose picked from the banks of the fountain of youth. These flowers are eternal, they never wither or die and when picked for a lover they are a symbol of everlasting love. They are also a key ingredient in the antidote for a great many supernatural poisons." A look of realisation swept across his face, "If I am correct then these flowers can only be found in _purgatory_."

"Very good Armin," Mikasa smiled, "To pass the exam all you have to do is_ pick_ one of these flowers."

"Well that doesn't sound too hard!"

Mikasa shook her head grimly, "The real challenge isn't picking the flower- it's the journey to get to it."

"I don't suppose we can travel there using angel feathers?" Eren asked hopefully.

Angel feathers are reaper transportation devices; one silver feather taken from the wing of an angel grants the user the power to travel _almost_ anywhere.

Mikasa frowned; "Heavenly items cannot access _any_ of the underworlds, including purgatory. If something is in purgatory then the only way to access it is through hell."

"But humans can't travel through hell alone!" Armin's face went white with fear.

"Then you'll have to find someone… or more accurately _something…_ to take you." Noticing Eren and Armin's horrified expressions; Mikasa added gently, "I'll be monitoring your life force from here. If one of you looks like you're in trouble, then I will come and rescue you."

Eren raised an eyebrow, "I thought that wasn't allowed."

Mikasa shrugged, "I once made a vow to protect you both with my life and I meant it. I will not let any harm come to either of you whilst you are on the field."

"But if that does happen and you try to come and rescue us…won't you just become trapped as well?"

"Purgatory flowers hold special properties; they allow the holders to travel to different worlds unencumbered - by that I mean they hold enough supernatural power that wearing one will allow travel safely through the underworlds. They also grant the wearer additional strength and speed." Mikasa smiled, "Given the amount of business we garner from demons, it makes sense that you need one of these flowers to become a fully-fledged reaper."

Eren took a deep breath. He had to do this. He had been training for three years to become a reaper and he wasn't going to fail now. Even if it meant storming the gates of hell and _literally_, he was determined to succeed.

"Time moves differently in the underworlds doesn't it Mikasa?" Armin asked, listening intently for any small piece of information that might give him an advantage.

Mikasa nodded, "When you return only a couple of days have passed in this world, although you may both feel as if you have been doing the test for years."

Eren gulped, "How long did it take you?"

"Using the underworld time-zone? About six months, but three years is probably the average."

They nodded and Mikasa was finally satisfied that the two of them were prepared and that they had no more questions. Taking each of them by the hand, she led them towards their respective grave stones.

"Here you both are," She gestured towards their head stones. Armin shuddered; he didn't like knowing that one day he really would be lying beneath it. "I asked to have both of yours put right next to mine." Mikasa smiled encouragingly at them before continuing. "Okay, final rule; you cannot confide in each other, from this point onwards you act alone. Once you are in hell you are not longer to contact earth-dwellers, human or not - not except in very exceptional circumstances. The death of a family member and war or a natural disaster in your home country are obvious exceptions."

"How will we know if any of these things happen?"

Mikasa reached into her pocket and handed the two boys each a mirror, "These mirrors contain the spirits of the merfolk who lost their bodies in an attempt to become human. We keep them _alive_ in the spirit realm in exchange for their services. In their current state they can travel between worlds and look upon them, although sadly without ever entering. Nonetheless they are quite capable of informing you if there is an emergency. These spirits will be your only window to the outside world."

"I'm going to miss you both so much," Armin said tearfully, pulling them both into a deep hug.

After they had finished saying their goodbyes, Mikasa lay he two boys down inside their coffins - but not before offering them one final piece of advice.

"Think not only of the easiest way to reach purgatory, but whom you could learn the most from. You will be spending an extended amount of time with whatever creature you choose. Please ensure it is not spent superfluously. Once you're sure of whom you wish to take you, you simply call out their name and they _should_ appear before you."

"_Should_?" Eren asked, wiggling to try and find a comfy position inside the coffin. He knew he was short, but he still felt like they had made his coffin just a _little too_ cushy.

"Whilst some creatures _delight_ in having a human indebted to them, others such as leprechauns and centaurs are less enthusiastic. If I were you I would use my training to ensure you choose something that will want to train you. Or at the very least tolerate your company."

"Jeez Mikasa, you could have told us all of this beforehand."

"Reapers are forbidden from discussing the entry exam." She shrugged, before continuing, " A final warning, once you enter the garden you are only permitted to pick one flower. Picking more than one will result in instance expulsion from the reapers and very probably your death. Only one reaper has ever been known to do so and survive."

"Cheers. That makes me feel much better."

"Just to clarify we can ask _any_ creature?" Eren heard Armin's voice from over the other side of him and smiled. Armin might not be strong - but he was smart. He clearly had a plan.

"You may ask anyone or anything you wish, just…be careful...certain creatures will try and trick you. It is fine to take a long time completing the exam, it is _not_ fine to lose your soul in the process."

Mikasa checked the graves one last time for malfunctions before taking the time on her watch.

"You boys ready?"

"Yes!" "...I guess so..."

"Then I'll start the exam. See you both on the other side... and good luck."

She gave them both a quick salute before reaching down on both sides and shutting the coffin doors.

As Eren heard the door slam behind him and he felt what he knew must be Mikasa lowering him deeper into the ground, Eren began to think.

Who, _or what_...should he choose?

A witch would be easy, he knew the names of quite a few light witches who would probably make him sweep the floors and bake for a couple of years.

Eren sighed, he didn't want that. What he wanted someone who could train him, someone that could make him _strong_.

That's when he realised, what he wanted was a _demon_.

Mikasa wouldn't approve, he knew that much. Demons were not only untrustworthy, but unlike imps and dark pixies they were _smart_. If he did this he would have to be very, very careful.

Uncomfortably Eren acknowledged to himself that he only knew the names of a few demons and only the very powerful ones. Any deal he decided to make was undoubtedly going to have a high cost…still, it was a risk he was willing to take.

"I call upon you," He began loudly, feeling a little embarrassed as he had never called on a demon for help before and therefore wasn't really sure how to address one. Still, he tried to sound as confident as possible, "Come to me from the sixth layer of hell, I call upon on you, the greater demon Rivaille!"

Rivalle was a incredibly powerful demon and lord of the beasts. Despite his high status he wasn't very popular amongst others of his kind, from what Eren had heard he mostly specialised in _slayer_ magic. That meant for the right price, he would do anything...even destroy one his own.

For a demon this was considered utterly unthinkable, but for a reaper...slayer magic could prove invaluable.

For the first couple of seconds nothing happened and Eren began to worry that he had done something wrong. He was just about to call out again when there was a loud crack of lightening and to his immense shock the coffin thrust upwards and opened.

When he stepped out he realised he was in a forest. Not a usual forest of course, the barks of the trees were red, the leaves were a violent mash of black and yellow and when he looked down Eren realised he couldn't see solid ground, just a thick layer of smoke.

"Well, well, well…what have we here?"

Eren swallowed and wiping the sweat from his brow, he turned around.

Dark sensuous eyes met his and Eren couldn't help notice a smirk appear on the demons cruel mouth. As it began to walk towards him Eren was startled by how gracefully the demon moved, if he hadn't seen otherwise he would have thought it was_ floating._

There was no doubting it, this was him. _The demon Rivaille_

Eren took a deep breath, he knew he needed to make the first move. He had to show this demon that he wasn't afraid.

"I am Eren Yeager of the reapers, I called you here to make a deal!"

The demon raised one of his thin eyebrows and smirked, "In that case I am the demon Rivaille...and _very much _at your service."

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**Author Note: First thing I've ever written so **_**any**_** feedback would be appreciated. **


	2. Chapter One

**Chapter One**

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**Book of Spirits, Page 24**

**Ser-vus [S**_**i**_**r**_**-**_**v**_**o**_**s]**

_**noun**_

Simply put, an owned human. Servus humans are those who have pledged their services to a demon, this can take the form of a single service (a debt or promise that can be fulfilled back on earth), or more unusually, direct servitude for a demon within one of the seven layers of hell.

There is no such thing as a 'typical' Servus job, they can range between mundane activities such as becoming a demons personal butler or cook, delivering a message or magical item back to earth and in more extreme cases; blood debts, usually requiring a sacrifice of some sort - in some cases from a human.

It is important to note, that whilst a demon may use a Servus for pleasure (whatever this might constitute for the individual demon) they must not develop an attachment. Whilst this is almost unheard of in greater demons, rarely a lower level demon, whose soul is still in the process of deterioration has found him/herself harbouring affection. This is not only considered a breach of demonic law, but if the soul begins to rebuild, can be fatal for the demon in question.

* * *

The demon Rivalle brought his legs up into the air and crossed them, looking at Eren expectantly.

"You can fly?" Eren asked, his mouth agape.

Rivalle snorted, "Angels fly; demons levitate."

Eren wanted to ask why that was, but resisted. He was trying to make a deal with this demon, seeming weak or stupid was not a good idea.

The demon smirked at him, before continuing "Angels will try and spool a load of shit about floating on the lords, light...but in _my_ opinion it's really just an issue of semantics."

Eren blinked, had the demon known he wanted to ask that? Could he read minds? His stomach tightened, maybe this wasn't such a good idea after all.

"You can read my mind?" Eren asked reluctantly, he wasn't delighted to reveal his ignorance, but if he was going to try and make a deal with this demon, he really had to know. Demons and reapers weren't directly rivals, but they did end up killing each other on a fairly regular basis. One of the reaper jobs was hunting down demons who tried to break their deals.

The demon tipped his head back in laughter, "no, but the question was written all over your face." He thought for a moment before continuing, "you used a coffin to travel here, did you not?"

Eren nodded

Rivalle raised an eyebrow, "then you're a reaper?" He sounded more than a little incredulous.

"Yes!" Eren responded indignantly.

"Standards in the overworld, really have taken a nosedive, haven't they?"

Eren, gritted his teeth as he felt his skin heat up. It was true, he wasn't the most well read in his class. He had focused so hard on combat training that he may have neglected his studies. He always assumed he would be working alongside Armin and Mikasa. Armin was the book guy and Misaka was all round brilliant, usually they just pointed him towards something and smashy-smashy, the job was done.

Instead of explaining all this, he just gritted his teeth, "Don't you insult me."

"Not an insult, merely an observation."

The demon uncrossed it's legs and lowered it's feet to the ground, before walking over to Eren. He never once removed his eyes from Eren, and his expression, which had previously been cool amusement, was now dark and predatory. He lifted his fingers and turned up Eren's chin, inspecting him closely.

Eren shifted uncomfortably on his feet, but did not move away. The demon smirked a little, "You may not have brains, but I cannot deny that you are a fine specimen of human beauty." He ran his finger down Eren's jawline and onto his neck, murmuring, "quite the prize."

Normally Eren would have objected, but he kept his temper in check. This demon was powerful, if he could convince him to train him, anything would be worth it.

"So," Rivalle asked, finally letting go of him. "What deal is it that you wish to make?"

Eren straightened his shoulders, _this _was it. Everything he had trained for came down to this moment.

"I need you to pick me a flower, from purgatory."

"Oh so you are a reaper in training." It wasn't a question, "and you need one of purgatory's golden roses and of course, my help to get it."

Eren nodded carefully.

"Interesting...and what could I hope for in return?"

"What would you want?" Eren asked cautiously. He may not be booksmart, but if he knew anything, it was that demons will try and trick you. The eternal laws might protect you from a deal breaker, but loopholes were a whole other matter. That was where demons did their best work. That was why they were so powerful.

The demon gave a chilly laugh, "a demon is only a demon by virtue of wanting _everything. _If we are to make this deal, I need to know what services you could provide for me."

"I…"

"I would suggest that you should have thought this through more carefully, but then what kind of demon would I be? No matter, you are a pretty specimen, I am sure you will be of value somehow…" he licked his lips.

The feeling of unease Eren had felt when the demon first arrived, was swiftly developing into full blown panic.

"You would need to train me as well." Eren said, trying to sound demanding.

The demon snorted, "any power you could ever hold, would only be a fraction of my own, so if you're hoping to 'fly'..."

Eren shook his head, "No, but I want to be more powerful."

The demon considered this for a while, before pulling a small, silver ring out his pocket and handing it to Eren. "Look at the front."

Eren gasped, it was a golden rose.

"So, you see my problem."

Eren nodded, "You can't own two purgatory flowers at once."

The demon nodded, "You may be a fool, but you do at least know that…" He considered for a moment, "I suppose I could just give you this one, but that would be too easy...and of course you couldn't be sure I was training you correctly…" As he said this something clearly clicked in his mind. "Okay Eren, I will train you and I will get you your flower."

"In exchange for…"

"You will become Servus, I will own you and you will obey me."

Eren frowned, "I won't kill for you, or -"

The demon laughed, "as if I would need you to kill for me. No, you will become my personal...attendant, shall we say. In return I shall train you, what would be acceptable training in human time, three times a week?"

"Yea that would be fine." he paused, "For how many years, would this be?"

"Well here's the thing. Whilst this flower exists, I cannot get you another one, so instead of a set period of time, I will train you until you have power enough to destroy this one."

"Is that even possible?"

Rivalle nodded, murmuring a curse under his breath and touching the flower lightly. Eren watched in amazement as one of it's golden petals floated to the ground."

"I cannot say how long it will take you to be able to do this, but when you do, your power within the reapers would be unmatched."

Eren's eyes widened. To be the best...better than Pixis, better than Marco, even better than Mikasa...anything was worth that, wasn't it?

"So I would be your servant…"

"_Servus,"_ He corrected, "...Until the last petal falls, yes. Do we have a deal?" He held out his hand.

Tentatively, Eren reached out and shook it. The demon laughed. "That is not how demon's seal deals," He pulled Eren's arm, dragging him forward, "_This_ is."

Rivalle leaned in towards him, taking his lower lip with his teeth and biting it gently, before sliding his tongue into his mouth. Eren gasped and he heard the demons low chuckle as he clasped a handful of Eren's hair and pulled their bodies closer together.

It was done.

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**Sorry for the lack of update, but me and my partner have had our hands very busy with our newly adopted baby girl! However, I have slightly more time on my hands now that she has settled down, so I am making my own pledge to update this story every two weeks. Chapters will vary in length depending on how much time I have, I know long chapters are preferable and will try to deliver 5k plus. But it might not always be possible. However, from now on, I promise I will try and deliver **_**something.**_

**I crave reviews, nearly as much as I crave Levi. Please, spare the time to validate me and my writing. :)**


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